This invention relates to an optical information processing apparatus and in particular to an optical information processing apparatus such as laser beam printer, optical disc device, optical card device, etc., which records and/or reproduces information by projecting a laser light spot on an information recording surface by means of a semiconductor laser used as a light source. More in detail it relates to an optical information processing device, in which the focal position of the laser light can be moved optically along the optical axis.
A prior art laser beam printer consisted of a gas laser such as an He-Ne laser, a light intensity modulator, a rotating polygonal mirror, an F.sub..theta. lens, and a light sensitive drum, in which laser light was focused by the F.sub..theta. lens on the surface of the rotating light sensitive drum and recorded information in the form of dots having a constant diameter on the light sensitive drum by modulating the light intensity of the spot by means of the light intensity modulator while scanning the surface of the light sensitive drum with the spot by means of the rotating polygonal mirror. Further, in a laser beam printer using a semiconductor laser as the light source, the emitted light intensity of the semiconductor laser was not modulated by the light intensity modulator but modulated directly in order to record dots having a constant diameter.
In an optical disc device and an optical card device, a laser light spot having a constant diameter was projected on the information recording surface of a rotating disc or a card so that circular pits having a constant diameter or elongated pits having a constant width were formed in order to record and/or reproduce information.
This invention relates to an improvement of the technique for displacing the focal position of the laser light along the optical axis in an optical information processing device, such as these laser printer, optical disc device, optical card device, etc. This technique is necessary for the focusing control to project a spot having a diameter always constant on the information recording surface by making the focal position of the light follow vertical movements (in the direction parallel to an optical axis) of the information recording surface. In addition, this technique is necessary for effecting multigrade recording or multivalue recording to record dots or pits having different diameters by projecting spots having diameters different according to information to be recorded on the information recording surface by displacing the focal position of the laser light to positions along the optical axis, which are different according to the information to be recorded. Heretofore the displacement of the focal position of the laser light along the optical axis was carried out by displacing mechanically a focusing lens along the optical axis by means of a lens actuator. This will be explained below, taking an autofocusing control in an optical disc device as an example.
An autofocusing control system in a prior art optical disc device consisted of a light source such as a semiconductor laser, a focusing lens, light beam separating means, such as a quarter-wavelength plate and a polarization light beam splitter, etc., a focal displacement detection optical system, and a focusing lens actuator. The focusing control was carried out, in order to reduce influences of vertical movements (fluctuations in the optical axis direction of the focusing lens) due to the rotation of the disc, when the light emitted by the semiconductor laser was focused by the focusing lens in the form of a spot on the surface of the disc, by separating light reflected by the disc by means of a quarter-wavelength plate and a polarization light beam splitter, etc., guiding it to the focal displacement detection optical system, detecting deviations of the focal position from the disc surface (focal displacement), driving the lens actuator by using detection signals thus obtained, and displacing only the position of the focusing lens along the optical axis.
The focal depth of the focused spot is given by .lambda./(NA).sup.2, where NA represents the numerical aperture of the focusing lens and .lambda. the wavelength of the laser light. For example, when NA=0.5 and .lambda.=0.83 .mu.m, it is 3.3 .mu.m. Consequently, when external vibrations and focal displacement detection errors are taken into account, focal displacement should be suppressed to about .+-.1 .mu.m by the lens actuator. On the other hand, the disc rotates usually at a speed of about 30 Hz with fluctuations of about .+-.200 .mu.m. Further there are small fluctuations in one rotation. For this reason, in a prior art autofocusing control system including a lens actuator, a high gain was necessary to suppress focal displacement to about .+-.1 .mu.m and in addition cut-off frequency characteristics as high as several kHz were necessary to make the focusing lens follow focal displacement of several 100 Hz. However, there was a problem that it is difficult to design and fabricate autofocusing control means including a lens actuator, because generally the cut-off frequency is low, when the gain is high. In particular, there was a problem that the cut-off frequency is determined principally by the lens actuator which moves mechanically the lens and therefore it is difficult to suppress also high frequency components of the focal displacement.
Similarly, when the focusing lens is moved under the mechanical control of lens actuator in order to effect multigrade recording, it is not possible to vary the diameter of the spot precisely with a high speed according to multigrade information. Therefore it was impossible to realize the multigrade recording, in which color is varied continuously or stepwise from black to white.